Soon I'm going to have to file a new nut. The slot on it are all standard .010 to .042 or whatever. I have .011, .013, .018, .028. 038, and .050.

I find file sets that are close but not right on. Is it possible to take a .026 file and work it side to side a bit to get a little extra width?

I want to get this project guitar exactly right.
If anything, go a touch larger. For example, for those gauges, go for 12, 15, 20, 40, 54, or something like that. It would help reduce friction a little and stop strings catching.